During the last decades the internal combustion engines have been developed towards an improved operation economy and a reduced environmental impact by the introduction of digital control systems for, for example, fuel injection and ignition.
However, in spite of said improvements, it is still a problem that the varying operation conditions result in the average efficiency being low and the effect of the environment being large. The composition of the exhaust gas emissions also varies, making it difficult to clean the exhaust gases.
It is well known that a Variable Compression Ratio (VCR) improves the efficiency. It is also well known that the introduction of freely operable valves, so called Variable Valve Actuators (VVA), results in the possibility of a change from throttle regulation to earlier closure of the inlet valves during the inlet stroke, so called Miller cycle, and the possibility of total cylinder closure during a time as short as one engine revolution, also called a Frequency modulated Torque or Skip Cycle, which results in a substantially improved efficiency. It is also a fact that it will be possible to change between 2-stroke cycles and 4-stroke cycles by the use of VVA.